Blob physical projection misreads empty non-null values

## Problem Blob columns can be misread when scanning with physical projection / binary blob handling if the column contains an empty but non-null blob value (`b""`). Empty blobs are encoded with `position == 0` and `size == 0`, which can be confused with rows that have out-of-line blob data in the decode paths. There is also a related lower-level scheduler contract issue: empty byte ranges should return an empty `Bytes` entry in result order, but the scheduler reassembly logic can drop empty ranges because empty intervals do not overlap any coalesced request. ## Impact After the first empty blob, subsequent blob values can be read back as empty or the scan can fail due to byte/result misalignment. ## Reproduction shape A blob column with values like: - non-empty blob - empty non-null blob - non-empty blob - null - non-empty blob Then scan it as binary with `blob_handling="all_binary"`. ## Expected The scan should preserve empty non-null blobs, nulls, and all following non-empty blob payloads in order.